The objective of Evolved UTRA and UTRAN is to develop a radio access network towards a high data rate, low latency, packet optimized system with improved system capacity and coverage. In order to achieve this objective, an evolution of the radio interface as well as the radio network architecture should be considered. For example, instead of using code division multiple access (CDMA) which is currently used in 3GPP, orthogonal frequency division multiple access (OFDMA) and frequency division multiple access (FDMA) are proposed air interface technologies to be used in the downlink and uplink transmissions respectively. One change is that all packet switched service in LTE, which means all the voice calls, will be made on a packet switched basis. This leads to many challenges in the LTE system design to support VoIP service.
While VoIP users can utilize the same benefit of advanced link adaptation and statistical multiplexing techniques that are used in the LTE system as data users, the greatly increased number of users that may be served by the system because of the smaller voice packet sizes can place a significant burden on the control and feedback mechanisms of the LTE system. The existing resource allocation and feedback mechanisms are typically not designed to deal with a large peak-to-average number of allocations, as found with VoIP users on the system.
Traditional cellular voice traffic has some distinguishing attributes, such as vocoder output at fixed intervals, well-defined maximum and minimum rates (output can be comfort noise, full rate, sub-rate, etc.), and traffic with a fractional voice activity factor (VAF) of approximately 35-50%. The VAF is a percentage of time that a user is speaking. It is desirable to exploit these attributes to schedule a large number of voice users with minimal resources for control signaling both for resource assignment and feedback.
Grouping users in similar radio conditions has been proposed by the prior art to reduce the amount of signaling and feedback required to support voice traffic in LTE. It was proposed that groups be associated with particular sub-frames to exploit the known traffic characteristics and thus allow for statistical multiplexing based on the VAF and retransmission requirements.
According to one prior art proposal, the network can exploit the VAF to overload the group with a user population larger than what a typical sub-frame would be able to support for the voice packets. For example, at 5 MHz it is expected that three or four users can be supported for voice services in a 0.5 ms sub-frame. With a typical VAF of 0.4, it should be possible to define a group of eight to ten users.
This general grouping principle is shown in FIG. 1, with users grouped based on their channel conditions. The rationale is that users in similar channel conditions would be supported with similar channel attributes such as modulation, coding rate, etc. FIG. 2 shows how to use a bitmap to assign radio resource blocks within a group of user equipments (UEs). Note that the group scheduling is a type of persistent scheduling with fast on/off control, which uses the preconfigured assignment of a smaller group of UEs to a sub-frame to reduce the number of addressable UEs in that sub-frame. As shown in FIG. 2, in a first sub-frame, radio resources are allocated to UE1, UE2, UE7, UE8, and UE9. If UE2 acknowledges a previous transmission and no longer needs the radio resource, that resource is free to be reassigned. In a second sub-frame, radio resources are allocated to UE1, UE3, UE7, UE8, and UE9.
The following problems are identified from the LTE system and existing proposals:
1. Detailed signaling options for grouping services are missing from current proposals.
2. The radio resources are allocated to the voice users on a semi-static basis. Due to the voice silent and active state transition, it is efficient to allocate the radio resources of a voice user to other users or services if that voice user is in the silent period. The Evolved UTRA Node B (eNB) can monitor the voice activity to all UEs to be transmitted in the downlink (DL) easily and make an efficient resource assignment and signal that decision to the UEs. If an eNB needs to re-allocate radio resources that are assigned to a UE for uplink (UL) voice service to other services or other UEs, there is a problem if this UE is in the voice silent period because the eNB cannot monitor the UL UE's voice activity and thus cannot make the efficient UL resource scheduling decision.
3. The channel condition is varying continuously and the radio resource block assigned to each UE for voice service cannot be in a fixed pattern, otherwise it will lead to performance degradation. Performance degradation can include, for example, a lost voice packet due to the deep fading channel if the assigned radio resource is fixed during the entire VoIP service. Consecutive lost voice packets can distort the reception by the listener, which can result in an unacceptable level of service. For example, when UE grouping is used, the radio resources assigned to UEs within a VoIP group cannot be in a fixed order. Some methods need to be proposed to change the resource allocation pattern and with relevant signaling to support the resource permutation.
4. If the UE grouping method is used for VoIP service to efficiently use the radio resources by utilizing the voice on-off activity to reduce the overhead, there will be a problem if the number of UEs within a group is too small. Considering the VAF, which is between 35-50%, if only ten voice users are grouped together as proposed in the prior art in one 0.5 ms sub-frame, it is not statistically correct that there are always less than or equal to four active users and six inactive users within one group. The system will have to assign new radio resources to UEs within a group if there are more UEs than available resources, which will cause extra overhead.
By considering the VAF, the grouping method can be used appropriately only if a large number of UEs are grouped together, so that it can be assumed that statistically there are around 35-50% active voice users and the rest are inactive voice users. So either more than ten UEs have to be grouped together or some resource scheduling methods have to be proposed. Also, having the same number of UEs within one group is not flexible to reflect channel and voice traffic volume.